This invention relates to capacitive liquid level sensors. Such liquid level sensors find use in many instruments wherein a robotic probe is used to withdraw liquid from a container holding a sample to be analyzed or a reagent.
In such robotic systems, it is desirable to have knowledge of the level of the liquid in the container such that the probe used to withdraw the liquid can be controlled to minimize contact with the contents of the container. In this manner cross contamination between samples and reagents is reduced and the job of washing the probe tip is made easy. In such robotic systems the probe is introduced into the liquid container and preferably maintained just below the surface of the liquid.
To accomplish this objective, various level sensors have been developed. Among those are the so-called capacitive level sensors. These are based on the fact that any conductor exhibits a finite electrical capacitance. When the probe actually touches a liquid, the higher dielectric constant and greater surface area of the liquid results in an increased probe capacitance. These capacitance changes can be rather small so that sensitive detection devices are required.
Devices known in the prior art that are suitable for detecting small changes in capacitance include bridges, RC or LC oscillators and frequency meter counters (including heterodyning), phase locked loops, zero crossing periodometers, amplitude changes to an RC or LC filter, and phase shift changes through an RC or LC circuit.
Among the prior art capacitive liquid level sensors is Kingston U.S. Pat. No. 3,391,547 which discloses a capacitive liquid level probe for a liquid tank. He utilizes a capacitor probe, disposed in the liquid, as one leg of a bridge circuit. An unbalance in the circuit, as a result of change in capacitance of the probe, is detected by a phase sensitive detector which is referenced by a fixed frequency excitation oscillator through a variable phase shifter. The variable phase shifter allows for offset adjustment.
In similar manner, Oberli U.S. Pat. No. 3,635,094, discloses a capacitive level sense means for an automatic transfer pipette. The sample probe is utilized as the first element and a metal stand around the sample vessel is the second element which forms a capacitor in one leg of a bridge circuit. The remaining legs of the bridge consist of a variable capacitor leg and two resistor legs. The variable capacitor leg may be adjusted such that its capacitance matches that of the probe contacting the liquid. The bridge circuit is excited by a fixed frequency oscillator and a differential amplifier is utilized to determine when the bridge is balanced indicating that the probe has contacted the liquid.
Bello et al. U.S. Pat. No. 4,326,851 discloses a level sense apparatus and method for use in an automatic clinical analyzer in which a variable capacitor is formed by a grounded probe and a metal plate, which is connected to the detection circuit, disposed below the sample vessel. A fixed frequency excitation signal is utilized and the capacitance change of the metal probe resulting from the probe contacting the liquid is detected as a voltage change in the detection circuit. This arrangement presents a problem in that spills on the electrode or supply tray can change the circuits operation and the circuit requires the use of shielding pads.
Another U.S. patent, Okawa et al. U.S. Pat. No. 4,736,638 discloses a liquid level sense apparatus for use in an automatic clinical analyzer. A metal plate disposed under the sample vessel and connected to a fixed frequency oscillator emits low frequency electromagnetic radiation up through the sample. The dispense probe serves as an antenna and is connected to a detection circuit, having appropriate bandpass filtering, which detects a voltage amplitude change when the probe contacts the liquid sample. This circuit has many of the disadvantages of Bello. In addition, the use of low frequency limits the time response of the circuit.
Finally, Shimizu U.S. Pat. No. 4,818,492 discloses a capacitive liquid level sensor for an automatic clinical analyzer. He utilizes a resistor bridge with a fixed frequency oscillator exciting one diagonal of the bridge and the probe serving as a capacitor across the other diagonal. Phase shift across the capacitor (probe), as a result of change in capacitance of the probe, is detected by a phase detector which is referenced by the fixed frequency excitation oscillator through a variable phase shifter. The variable phase shifter allows for offset adjustment. The output of the phase detector is filtered and compared against a reference value to provide a signal indicating the presence of liquid at the probe.
The problem with many of these prior sensors is that they have to be run at relatively high frequencies in order to have the sensitivity required to detect liquid levels. This causes them to be sensitive to changes in capacitance of the flexible plastic tubing that couples the sucking probe to the suction pump. Hence the simple motion of the tubing as the probe is raised and lowered causes false levels to be indicated. This limits the sensitivity of the probe.